The body maintains steady-state balance for water and a number of important solutes. This occurs when input into the body equals output from the body. For each solute and water, there is a normal set point. Deviations from this set point are monitored (i.e., when input ≠ output), and effector mechanisms are activated that restore balance. This balance is achieved by adjustment of either intake or excretion of water and solutes. Thereafter, input and output are again equal to maintain balance.
The Na+,K+-ATPase and K+-selective channels are critically important in establishing and maintaining the intracellular composition, the membrane potential (Vm), and cell volume. Na+,K+-ATPase converts the energy in
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ATP into potential energy of ion gradients and the membrane potential. The ion and electrical gradients created by this process are then used to drive the transport of other ions and other molecules, especially by solute carriers (i.e., symporters and antiporters).
• Epithelial cells constitute the interface between the external world and the internal environment of the body. Vectorial transport of solutes and water across epithelia helps maintain steady-state balance for water and a number of important solutes. Because the external environment constantly changes, and because dietary intake of food and water is highly variable, transport by epithelia is regulated to meet the homeostatic needs of the individual.
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3 Signal Transduction, Membrane Receptors, Second Messengers, and Regulation of Gene Expression
Upon completion of this chapter, the student should be able to answer the following questions: 1.
How do cells communicate with each other?
2.
What are the four classes of receptors, and what signal transduction pathways are associated with each class of receptors?
3.
How do steroid and thyroid hormones, cyclic adenosine monophosphate, and receptor tyrosine kinases regulate gene expression?